Since you were using expr, I assumed that you were using sh which cannot do arithmetic. ksh is a great shell, but you're not using all of its features yet.

With ksh you can do:

((percent = 100 * used / avail))

which will work the same as

((percent = 100 * $used / $avail))

if "avail" and "used" both contain simple numbers.

I'm not infront of a box I can test but it seems logical to me that printf or one of it's variants may have the thousands comma separator functionallity Neil was looking for. If anyone knows for sure, post back otherwise I will try to figure it out tomorrow if I have time.

That's pretty awesome. But it's more powerful than the comments seem to indicate. It handles leading minus signs. It also handles a decimal point followed by up to three decimal places. It not obvious how to handle commas with more than three decimal places. Leave them alone? Like this: "1,234,567,890.0000" ?

If that is right, we can extend the sed one-liner:

sed -e :a -e 's/^\([^.]*[0-9]\)\([0-9]\{3\}\)/\1,\2/;ta'

This countinues to handle leading minus, plus, and dollar signs. In the accounting world, I've seen an odd convention of putting numbers in parentheses. That also works.